Radiation thickness gauges using radiation have been used as main apparatuses for measuring the thickness of a plate rolled by a rolling mill. These radiation thickness gauges include X ray thickness gauges using X rays and y ray thickness gauges using y rays. The radiation thickness gauges have an ensured thickness measurement accuracy of 0.1% and have been used for quality control for normal plate thicknesses.
However, the radiation thickness gauge fails to focus a beam system for radiation and thus provides an insufficient spatial resolution. Furthermore, the radiation thickness gauge needs a process for eliminating the adverse effect of noise, leading to a commensurate reduction in response speed. Thus, the radiation thickness gauge has difficulty detecting variations in plate thickness which are long in the width direction of the plate to be rolled and which occur at a constant pitch in the traveling direction, such as chatter marks resulting from mechanical vibration of the rolling mill or roll marks resulting from deformation of or damage to the rolling mill.
That is, detection of chatter marks or roll marks needs a thickness measurement accuracy of 0.1% or several micrometers or less in absolute value. Furthermore, a resolution is needed which allows the varying speed and shape of the plate being moved to be dealt with. Specifically, the detection needs a measuring spatial resolution of 10 mm and a response speed of 1 msec. or less. Thus, for thickness measurement demanding a high resolution, a thickness measurement apparatus 200 based on a laser range finder utilizing laser light as shown in FIG. 5 is used instead of the radiation thickness gauge.
The thickness measurement apparatus 200 includes a C-shaped frame 15 and a thickness calculation section 113. The C-shaped frame 15 includes an upper arm portion 15T and a lower arm portion 15B equipped with a first laser range finder 11 and a second laser range finder 12, respectively. The first laser range finder 11 and a second laser range finder 12 are arranged so that a measurement surface of the first laser range finder 11 lies opposite a measurement surface of the second laser range finder 12. The distance between the measurement surfaces of the first laser range finder 11 and the second laser range finder 12 is denoted by Lo. The C-shaped frame 15 is disposed such that the measurement surfaces of the first laser range finder 11 and the second laser range finder 12 lie opposite a top surface and a bottom surface, respectively, of a measurement target (a plate moved while being rolled) 5. The first laser range finder 11 and the second laser range finder 12 each include a laser light source section, a CCD camera, and a distance calculation section (none of which are shown in the drawings). Laser light output by the laser light source section is delivered to a surface of the measurement target 5. Reflected light from the measurement target 5 is received by the CCD camera, which converts the light into an electric signal. The electric signal is input to the distance calculation section. The distance calculation section detects a change in the input electric signal, and based on the detection result, calculates and determines distances La and Lb between the measurement target 5 and the measurement surfaces of the first laser range finder 11 and the second laser range finder 12. The distance calculation section then sends the determined distances La and Lb to a thickness calculation section 113, which executes a calculation in accordance with Formula (1) to determine the thickness t of the measurement target 5.t=Lo−(La+Lb)   (1)
However, disadvantageously, in the thickness measurement apparatus 200, when the distance Lo between the arm portions 15T and 15B of the C-shaped frame 15, to which the first laser rangefinder 11 and the second laser range finder 12 are fixed, is varied by a change in ambient temperature, the variation itself corresponds to a measurement error as indicated by Formula (1). Thus, effort has been made to improve the structure and material of the C-shaped frame 15 in order to suppress a variation in distance Lo. Furthermore, various calibration methods have been proposed which are intended to deal with a temperature drift caused by the variations.
A method for the thickness measurement apparatus 200 involves performing calibrations at intervals (for example, every 5 seconds or shorter) smaller than a time rate of change for thermal shrinkage of the C-shaped frame 15 so as to avoid affecting production of the measurement target 5.
Another method for the thickness measurement apparatus 200 uses a chatter mark detection apparatus. The chatter mark detection apparatus includes both a thickness measurement apparatus 200 based on a laser with a high resolution and a thickness gauge based on radiation and which is unsusceptible to a variation in distance. The chatter mark detection apparatus determines, as a correction value for the temperature drift, the difference between an output from the radiation thickness gauge and an output from the laser thickness measurement apparatus 200 to correct the output from the thickness measurement apparatus 200 based on the correction value.
One known laser thickness measurement apparatus needs a large swing arm located at a measurement position on a production line on which a measurement target is conveyed, to set a calibration sample in a short time. This disadvantageously leads to an increase in the size of a calibration apparatus. The laser thickness measurement apparatus also needs to set a state in which no measurement target is present. This disadvantageously results in the need to halt the production of the measurement target, though the halt lasts only a short time.
Other known thickness measurement apparatus enables continuous measurement and solves the problem of the halt of the production. However, the thickness measurement apparatus uses radiation and thus needs special measures for safety management for radiation, for example, provision of a safety management zone.